1. Field of the Invention
The present invention generally relates to a reproduced video signal processing apparatus and, more particularly, is directed to a slow motion reproducing appratus for reproducing a video signal from recording tracks which are formed obliquely on a magnetic tape so as to be sequentially arranged in the longitudinal direction thereof.
2. Description of the Prior Art
A video tape recorder (VTR) of helical scan type is known as the slow motion reproducing apparatus of this kind. For example, in the proposed VTR for recording and/or reproducing a high definition television video signal, the television video signal is separated into four component signals (G1, G2, B and R signals) and these four video signals are sequentially recorded on adjacent four tracks T1 to T4 simultaneously as shown in FIG. 1.
In order to reproduce the respective component signals from such recording tracks, four playback heads assigned to the tracks T1 to T4 are tracked to trace the corresponding tracks simultaneously so that a composite video signal must be reproduced by composing the reproduced video signals derived from the respective playback heads as required. To this end, the VTR is so formed that if while the tape is transported, for example, at normal speed, the tape is reproduced under normal mode and after each playback head traces the corresponding track once from its lower end edge (tracing start end) to its upper end edge (tracing stop end) toward the head tracing direction, it traces the succeeding track, it is tracked to trace the lower end edge of the track formed at the position distant apart by 4 tracks from the above traced track.
Some VTRS for reproducing an information recorded on a magnetic tape by a plurality of heads have a so-called shuttle mode in which while the tape running direction and the tape running speed are both varied, a picture is reproduced. In this case, if the tape is transported at other speed than the normal speed (that is, the same speed as the tape speed upon recording mode), the tracing locus drawn on the magnetic tape by the position of the playback head attached to a magnetic drum does not coincide with the angle of each track in its longitudinal direction and crosses each track.
More particularly, in the shuttle mode, upon normal playback mode in which the tape is transported at normal tape speed, each head traces the track assigned to each head (this is called a corresponding track) as shown by reference numeral TRN in FIG. 1. However, upon playback mode with variable tape speeds in which the tape is transported at tape speed, for example, twice the normal one in the forward direction, the head draws a tracing locus TR(+2) which crosses four tracks in the direction opposite to the tape transport direction by the trace; upon still mode in which the tape stops running, the head draws a tracing locus TRS which crosses four tracks in the width direction of the tape; and upon reverse mode with tape speed 1/2 times the normal tape speed in which the tape is transported at, for example, tape speed 1/2 times the normal tape speed in the reverse direction, the head draws a tracing locus TR(-1/2) which crosses six tracks in the direction opposite to that of the normal playback mode.
In practice, in the shuttle mode, the tape running speed can be varied continuously so that it is necessary that when any tracing locus is designated over a range from the tracing locus TR(-1/2) of the reverse mode with tape speed 1/2 times the normal tape speed to the tracing locus TR(+2) of the playback mode with tape speed twice the normal tape speed, each head is tracked on the corresponding track correctly whereby to reproduce the component video signal without any dropout.
In practice, each component video signal thus reproduced is stored once in a field memory provided in a reproduced video signal processing apparatus (for example, a time base corrector), then read out at a predetermined timing and then composed as the composite video signal.
A dynamic tracking head (hereinafter simply DT head) is used to enable each reproducing head to be tracked on the corresponding track by which the head is displaced in the direction perpendicular to the tracing direction by the displaced amount of the tracing locus of the head attaching position from the corresponding track.
However, when the plurality of, for example, four playback heads, discribed in connection with FIG. 1, are tracked by using the DT heads to trace the corresponding tracks in the shuttle mode, the displaced amount of the DT heads become extremely large, while the displacing amount of the DT head, which can be manufactured in practice, is limited by a relatively small amount. As a result, it is very difficult for the plurality of heads to carry out the dynamic tracking operation simultaneously by using such DT heads the dynamic tracking function of which is limited.
For instance, the track width of the corresponding track of each playback head is about 60 .mu.m and the track pitch becomes around 357 .mu.m. Whereas, in order to carry out the dynamic tracking playback over the range from the still mode to the variable speed playback mode with the tape speed twice the normal tape speed, it becomes necessary to provide a DT head whose movable range covers more than 714 .mu.m as a peak-to-peak value. However, the range in which the displacement amount (the stroke of the peak-to-peak value) of the DT head provided in practice is linear is around 700 .mu.m. If the displacement amount exceeds this range, the stability of the dynamic tracking operation is lost. Accordingly, it is not considered practical to make the DT head carry out the dynamic tracking operation in one tracing over a full range from the still mode to the variable speed playback mode with tape speed twice the normal tape speed.
In addition, as the drive mechanism (generally a bimorph leaf is used) for the plurality of (four) heads to carry out the dynamic tracking operation, it is necessary to provide such a bimorph leaf which has the strength corresponding to the weight of the head, the resonance frequency and the flyback time upon the jump mode. It is, however, difficult to obtain such ideal bimorph leaf in practice.